I Have recently acquired an Oric Atmos because of nostalgia reasons
However, I tried to load cassette games from a cassette deck and none of them fully worked. One game loaded partly, with working menus but freezing when starting the game.

I would like a simple answer how to connect my PC to oric for reliable audio connection. Can I buy some cable?

You can directly connect the audio output of your PC to the tape input of the Oric, this will work but you have to be careful else you may have non funny results

So, important things to remember:
- Make sure your Oric is plugged on the same power plug as your computer. Some people have experienced electric shorts due to differential between separate electric circuits, but when both machines share the same power-plug things seem fine.
- To allow the Oric to load, you need to play a WAV file, there are some tools to convert TAP archives to WAV files like Tap2Wav and Tap2CD (can find them here: http://oric.ifrance.com/index_english.html)
- As on a normal tape deck, you have to make sure you find the correct volume. To high or to low and it will not work. Same thing if you enable special bass boost effects, reverb and other fancy parameters of modern sound cards: Pure simple sound is the best.
- Disable all your PC sound schemes, and don't run applications that love to play "beeps", "clicks" and other "ting dounnnggg" sounds, the Oric will receive them as well

And of course this works in both direction, you can record what you Oric send, and save it as a WAV file and convert it to TAP.

A few additions to Dbug's answer:
- rather than oric.ifrance.com, try oric.free.fr. It's the same site, but it holds more recent updates.
- using TAP2WAV, be sure you are using the "-11" switch to produce 11khz files. 8khz files are not read correctly enough by Windows to load on the Oric

Dbug wrote:
- Make sure your Oric is plugged on the same power plug as your computer. Some people have experienced electric shorts due to differential between separate electric circuits, but when both machines share the same power-plug things seem fine.

Plugging both Oric and Computer into the same plug may blow whatever fuse you have in the plug (from an English perspective).

When you refer to the Power plug i think you are referring to the extension lead block or wall socket. I think the issue is that plugging into two seperate wall sockets(or two seperate extension leads) may not link earth or nuetral so well.
I reckon most people will posess a Power extension lead and therefore the computer should plug into the same extension lead block as the Oric.

What went wrong? No "Loading"? Or program hanging?
If the programs hang, did you check they are not Oric-1 versions? (testing with an emulator with ROM v1.1)
These are old programs and they exist in several versions. The first ones are Oric-1 only!

If there's no "Loading", did you double-check the -11 switch?
Also, you could try using another program than Winamp. IIRC, it sometimes "eats" the very end of the file, which is not a problem with music but keeps the Oric in "Loading .." state because of the missing last bytes!
Another solution is to add a small silence at the end of your WAV file after the Oric program.